Hardening of steel articles



to be added can only be determined by periodic 50 they have attained the temperature of the bath,

UNITED STATES 'PATENT'OFFICE HARDENING or STEEL ARTICLES Harold Bradley, Frodsham, England, assignmto Imperial Chemical Industries Limited, a cor poration of Great Britain No Drawing. Application March 28, 1940, Serial No. 326,467.; In Great Britain April 4, 1939 9 Claims. (Cl. 148-15) This invention relates to hardening steel arcommence operation with a fresh charge of fused ticles, and more particularly to hardening arsalts. ticles of high carbon steel. It is an object of the present invention to It is well known to modify the characteristics provide an improved salt bath for treating high of steels by immersing them in baths of molten carbon steel articles. It is a further object of salts. Thus it is known 'to caseharden articles the invention to provide an improved salt bath of iron, steel and alloys thereof by immersing for treating high carbon steel articles which can them in a molten bath containing in addition to be maintained in a satisfactory condition for a cyanide a mixture of alkali metal and/or use for much longer periods of time. It is a alkaline earth metal chlorides, and also an alkali 10 still further object of my invention to provide metal carbonate. Again molten salt baths, such an improved process for hardening high carbon as fused mixtures of alkali metal and alkaline steel articles. Other objects of the invention earth metal chlorides have been used to effect will be apparent hereinafter.

purely physical changes in the properties of iron I have found that by operating with baths or steel articles, as for example high carbon containing a high carbonate content it is possteels. Thus high carbon steels, i. e. steels consible to omit the separate step of withdrawing taining more than approximately 0.6% carbon, part of the salt bath periodically prior to making are worked in a soft condition, and in order to up the volume of the bath, the accumulation of harden them subsequently they may be heated carbonate being prevented solely by the dragfor a suitable time in such a fused salt bath out losses; moreover, in spite of what would maintained at a suitable temperature and then have been expected from the prior art, I have quenched by immersion in brine, or otherwise. not found that the high carbonate content has Usually small amounts of cyanogen compounds, caused decarburisation to occur. It is thus pose. g. alkali metal cyanides are also added to the sible to ,mainta n t Composition a d e V01- bath to prevent superficial decarburisation of ume of the bath merely by adding the approthe articles during hardening, and baths have priate mixture of chlorides in the ratio in which also been described which in addition contain they are present in the bath, with a small amount small amounts of alkali metal carbonate to inof cyanide to compensate for the loss through crease its fluidity at a given temperature. More decomposition. Moreover, the bath can be mainthan small amounts of carbonates have, howtained in a usable condition for much longer ever, been considered deleterious because they periods than-hitherto. were considered liable to bring about decarburi- According to the present invention, therefore, sation. a fused salt bath for treating high carbon steel In use such baths tend to alter in composition articles consists of a mixture molten at the temthrough decomposition of the cyanide resulting perature of operation and containing between 1 in an accumulation of unwanted carbonate and and 10% by weightof an alkali metal cyanide also to decrease in volume through drag-out and between and by weight of an alkali losses, metal carbonate, the remainder being one or In order to maintain the bath at a comparamore alkali metal chlorides and/or alkaline tively constant composition it has been necessary 40. earth metal chlorides. Suitably the alkali metal to withdraw quantities of the fused saltbath carbonate is sodium carbonate, and the mixture in addition to the drag-out losses and to make of chlorides consists of two or more of the group up the volume of the bath by the addition of consisting of sodium, potassium, barium and calmixed chlorides, thereby readjusting the carcium chlorides. I bonate content to its initial small value; in ad- 45 According to a further feature of the invendition it has been necessary to add quantities of tion I provide a' process for hardening high carcyanide from time to time to make up for that bon steel articles, i. e. steel articles containing decomposed to carbonate. The quantities of the at least 0.6% carbon, which comprises immersmixed chlorides and of the cyanides which have ing the articles in such a fused salt bath until analyses of the fused salt bath and thus almost and then quenching the articles, e. g. in brine. daily skilled supervision is required for a process A typical bath consists-of 5% sodium cyanide, which otherwise could be carried out by an un- 67% sodium carbonate, 7% sodium chloride, 7% skilled operative. Even so it becomes necessary potassium chloride and 14% barium chloride. in time to reject the whole of .the bath and re- (:5 In using such a bath it is maintained at a suitable temperature for hardening high carbon steels, e. g. between 750 and 850 C. or preferably between 780 and 800 C. and the articles are immersed in the bath until they have attained the temperature of the bath and are then withdrawn and quenched in e. g. brine.

For small articles such as filesthe necessary time of immersion in the bath may be only a matter of seconds, e. g. 40 to 80 seconds while in other cases, depending on the size of the articles, times of the order of 2 to 3 minutes are suitable; in the case of large articles still longer times may be necessary. For the subsequent quenching brine is particularly suitable and articles with a clean dull appearance free from adherent salts are obtained. Other quenching media however, may be used satisfactorily, such as aqueous solutions of caustic soda, sodium cyanide, sodium carbonate and potassium carbonate.

In contrast to many cyanide-containing fused salt baths used for heat treating iron and steel and alloys thereof, it is possible to maintain the constitution and volume of the salt baths according to my invention by the addition of a single salt mixture. In using fused salt baths drag-out losses invariably occur, and in addition where such baths contain a cyanide, the cyanide content drops through decomposition to carbonate.

In baths according to the present invention, the relative rates of accumulation of carbonate through decomposition and of diminution through drag-out losses are of the same order, and the bath can, therefore, be maintained substantially at its original composition for much longer than has hitherto been possible by the addition of a mixture of alkali metal chlorides and/or alkaline earth metal chlorides in the proportion in which they are present in the bath, with' an amount of the alkali metal cyanide. The relative amounts 7 of the cyanide and of the chlorides in the mixture will be determined by th rate at which the former is lost through decomposition and the latter through drag-out losses.

The composition and amount of the salt mixture which is used for making up the bath will depend on the use which the bath is given, and the rate at which cyanide is decomposed, but for normal usage it may be taken that drag-out losses may be of the order of between 1 and 2% of the weight of the bath each day, and the daily rate of cyanide decomposition will be approximately the same. The composition and volume of the original bath, can thus generally be restored by adding a salt mixture containing the percentage of alkali metal and/or alkaline earth metal chlorides present in the bath, the remainder being alkali metal cyanide. Thus a bath having the composition NaCN, 67% NazCOs and 28% alkali metal and alkaline earth metal chlorides can be maintained at a substantially constant composition by replacing the drag-out by a mixture containing 28% chlorides in the proportion in which they are present in the bath, the remainder being sodium cyanide.

I claim:

1. A fused salt bath for heat-treating high carbon steel articles consisting of a mixture molten at the operating temperature and containing between 1 and 10% by weight of an alkali metal cyanide and between 60% and 75% of an alkali metal carbonate, the remainder being at least one salt selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides.

2. A fused salt bath for heat-treating high carbon steel articles consisting of a mixture molten at the operating temperature and containing between 1 and 10% by weight of an alkali metal cyanide and between 60% and of sodium carbonate, the remainder being at least two salts selected from the group consisting of sodium chloride, potassium chloride, barium chloride and calcium chloride.

3. A fused salt bath for heat-treating high carbon steel articles consisting of a mixture molten at the operating temperature and containing 5% sodium cyanide, 67% sodium carbonate, 7% sodium chloride, 7% potassium chloride, and 14% barium chloride.

4. A process for hardening high carbon steel articles which comprises the step of immersing the articles in a fused salt bath molten at the hardening temperature and consisting of between 1 and 10% by weight of an alkali metal cyanide and between 60% and 75% of an alkali metal carbonate, the remainder being at least one salt selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides,

withdrawing the articles when they have attained the temperature of the bath, and quenching the articles, e. g. in brine.

5. Process according to claim 4 in which the fused salt bath consists of between 1 and 10% by weight of an alkali metal cyanide and between 60% and 75% of sodium carbonate, the remainder being at least two salts selected from the group consisting of sodium chloride, potassium chloride, barium chloride and calcium chloride.

6. Process according to claim 4 in which the fused salt bath consists of 5% sodium cyanide, 67% sodium carbonate, 7% sodium chloride, 7% potassium chloride, and 14% barium chloride.

7. Process for hardening high carbon steel articles which comprises immersing the articles in a fused salt bath maintained at between 750 and 850 C. and preferably between 780 and 800 C. said bathconsisting of between 1 and 10% by weight of an alkali metal cyanide and between 60% and 75% of an alkali metal carbonate, the remainder being at least one salt selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, withdrawing the articles when they have attained the temperature of the bath, and quenching the articles e. g. in brine.

8. In a process for hardening steel articles which comprises maintaining at hardening temperature a fused salt bath molten at that temperature and consisting of between 1 and 10% by weight of an alkali metal cyanide and between 60% and 75% of an alkali metal carbonate, the remainder being at least one salt selected from the group consisting ofalkali metal chlorides and alkaline earth' metal chlorides, the step which comprises maintaining the volume and composition of the bath by intermittently adding to the bath a salt mixture containing alkali metal and alkaline earth metal chlorides of the kind present in the said bath, and in the proportion in which they occur therein, the remainder of the salt mixture being an alkali metal cyanide.

9. Process according to claim 8, in which the salt bath has the composition 5% sodium cyanide, 67% sodium carbonate, and 28% alkali metal and alkaline earth metal chlorides, and the salt mixture added contains 72% sodium cyanide.

HAROLD BRADLEY. 

